Do masks reduce viral transmission, how effective are they?
The primary mode of transmission between humans for SARS-CoV-2 is direct or indirect contact, respiratory droplets (diameter >5 µm), and fine-particle aerosols (diameter ≤5 µm). Individuals with no obvious signs or symptoms can also transmit the virus through droplets and aerosols (subclinical infection period accounted for 44% of infections).1
With healthcare authorities recommending the use of face masks to decrease COVID-19 transmission, little is known about the filtering efficacy of surgical face masks in respiratory viruses and viral release reduction.
A study published in the beginning of April 2020 has explored the transmission and routes of different respiratory viruses in patients with acute respiratory virus illnesses, with and without surgical face masks.2
The main findings showed that, in symptomatic patients, surgical masks reduced the detection of coronavirus viral copies in both large respiratory droplets and in aerosols. However, there was a reduction in the spread of influenza virus particles in respiratory droplets but not in aerosols.2
Not all individuals exposed to SARS-CoV-2 will become infected and not all infected patients will develop severe respiratory illness. Around 15% of infected patients will however progress to the severe phase of the infection.
SARS-COV-2 infects the host cell by binding to ACE2 receptors through its Spike proteins (S-protein). Therefore, cells expressing ACE2, as well as TMPRSS2, are susceptible targets.
These include different cell types, mostly in the upper and lower respiratory tracts2,3 (nasal gobelet, 3 ciliated cell, 3 sinus epithelium4, bronchial epithelium, 5 pneumocytes type II, 4 alveolar macrophages, 4 alveolar epithelial cell I, 3 bronchial transient secretory cells) 5 the gastrointestinal tracts3 as well as the liver and kidney.6
The infection can trigger innate and adaptive immune responses. In patients with severe COVID-19, lymphopenia is common along with drastically reduced numbers of CD4+ T cells, CD8+ T cells, B cells and natural killer cells, as well as a reduced percentage of monocytes, eosinophils and basophils. 7
An increase in neutrophil count and in the neutrophil-to-lymphocyte ratio usually indicates higher disease severity and poor clinical outcome.
In parallel, severe COVID-19 cases mostly present with significantly elevated serum levels of pro-inflammatory cytokines including IL-6 and IL-1β, as well as IL-2, IL-8, IL-17, G-CSF, GM-CSF, IP10, MCP1, MIP1α (CCL3) and TNF, designated as cytokine storm.
A cytokine storm may lead to shock and tissue damage in the heart, liver and kidney, and respiratory failure or multiple organ failure. Moreover, the pro-inflammatory cytokines mediate extensive pulmonary pathology, leading to massive infiltration of neutrophils and macrophages, diffuse alveolar damage with the formation of hyaline membranes and a diffuse thickening of the alveolar wall. Immune-mediated damage manifesting as spleen atrophy and lymph node necrosis has also been observed in deceased patients. 7
Given the fact that individuals may or may not present symptoms, and in the case of the latter a high overlap with other respiratory diseases, diagnosing COVID-19 based solely on clinical symptoms is inaccurate and must be complemented with either a PCR or immunoassay test.
The scientific efforts put into sequencing and publishing the entire genome of SARS-CoV-2 so quickly made it possible to design primers and use RT-PCR in diagnosis. Only a small amount of RNA, usually through a pharyngeal swab, is needed for an accurate result within hours. One set back however is the false-negative rates as the sample needs to be of high quality with intact RNA in order to work. 8
Since antibodies and antigens are much more stable than RNA, there is considerably less chances of having false negatives in immunoassays. Through sputum or blood samples, a result can be delivered in less than an hour.
Another advantage of immunoassays is that they provide historic information about viral exposure. Viral RNA is detectable only during the early stages of infection and are cleared once the patient has recovered from COVID-19. The type of antibody and its levels on the other hand can reflect the stage of infection and time since exposure. Available data shows that initial IgM antibody response doesn’t peak until around day 9 of infection and IgG antibody at around day 11. 9 Albeit slow to respond, antibodies can persist in the bloodstream after infection and therefore shed light on the immune status of an individual and whether they are susceptible to infection – a powerful tool for wide-scale public health measures.
Graphics design & creation by Elodie Gazquez